© National Instruments Corporation 5-1 SCXI-1120 User ManualChapter 5ProgrammingThis chapter contains a functional programming description of the SCXI-1120 and Slot 0.Note: If you plan to use a programming software package such as NI-DAQ, LabWindows, orLabVIEW with your SCXI-1120 board, you do not need to read this chapter.Programming ConsiderationsProgramming the SCXI-1120 involves writing to the Configuration Register. ProgrammingSlot 0 involves writing to the HSCR and FIFO Register. Programming the data acquisitionboards involves writes to their registers. See your data acquisition board user manual for moreinformation. The programming instructions list the sequence of steps to take. The instructionsare language independent; that is, they instruct you to write a value to a given register withoutpresenting the actual code.NotationFor the bit patterns to be written, the following symbols are used:0 binary zero1 binary oneX don't care; either zero or one may be writtenC one of three bits used to specify the channel to be loaded into the MUXCOUNTER. Thisvalue will either be the channel to be read for single reads, or a starting channel forscanned measurements.The 16-bit patterns are presented MSB first, left to right.Register WritesThis section describes how to write to the Configuration Register, HSCR, and FIFO Register,including the procedure for writing to the Slot-Select Register to select the appropriate slot. Fortiming specifics, refer to the Timing Requirements and Communication Protocol section inChapter 2, Configuration and Installation. The rear signal connector pin equivalences to thedifferent National Instruments data acquisition boards are given in Table 5-1. Also seeAppendix E, SCXI-1120 Cabling. The Configuration Register, the HSCR, and the FIFO Registerare write-only registers.The different bits in these registers often control independent pieces of circuitry. There are timeswhen you may want to set or clear a specific bit or bits without affecting the remaining bits.However, a write to one of these registers will affect all bits simultaneously. You cannot readthe registers to determine which bits have been set or cleared in the past; therefore, you shouldmaintain a software copy of these registers. You can then read the software copy to determinethe status of the register. To change the state of a single bit without disturbing the remainingbits, set or clear the bit in the software copy and write the software copy to the register.
